A conventional MOSFET operates by driving current through a channel region between the source and drain of a device. The conductivity of the channel region is modulated by the application of a voltage on the conducting gate above the channel surface and insulated from it. Efforts are ongoing within many MOS integrated circuit manufacturing companies as well as at many universities and government laboratories to improve the speed and available drive currents with MOSFETs to reduce their power consumption, and to improve their reliability and radiation hardness for applications in harsher remote environments, including space.
FIG. 1 shows a conventional partially depleted SOI (silicon-on-insulator) MOSFET that has been provided to achieve some of the improvements in speed and drive currents that have been needed. The OSI transistor 10 includes a silicon substrate 12 on which a buried oxide layer 14 is provided. A body layer 16, made of silicon, forms the area in which the semiconductor devices are located. The SOI transistor 10 includes a source region 18, a drain region 20 and a gate 26 that is provided on a gate oxide layer 22. Spacers 24 are formed on the sidewalls of the gate 26 and are employed as masks during the source/drain implantation process.
One of the concerns of a traditional partially depleted SOI MOSFET, such as the SOI transistor 10 of FIG. 1, is the decrease in the threshold voltage Vt of the transistor 10 due to hot carrier effects. As is well known, hot carrier effects in a transistor generate electron/hole pairs. Driven by electric fields, the electrons drift towards the gate 26, while the holes tend to drift toward the buried oxide layer 14. This movement of the holes toward the buried oxide layer undesirably decrease the threshold voltage Vt of the transistor 10.
A plot of Ids vs. Vds is shown in FIG. 2 for a conventional SOI MOSFET transistor 10, such as that depicted in FIG. 1. As can be readily appreciated, the well-known “kink effect” as depicted in FIG. 2, is due to the holes that have drifted near the buried oxide, the uncontrolled kinking increasing the substrate bias and thereby decreasing the threshold voltage Vt.
One of the goals in semiconductor processing is to maximize the use of the available silicon area. This allows increased miniaturization of the electronic circuitry. In particular, it is desirable to maximize the drive current for a given silicon area.